In the state of the art, it is known to evaluate the oscillations of mechanically oscillatable units for determining and/or monitoring the fill level of a medium. To this end, for example, a tuning fork with two fork tines, or, for example, a single rod, is excited to oscillate mechanically. When there is coupling between the fork and the medium, characteristics of the oscillations change: the amplitude decreases, the frequency becomes smaller, or a phase shift occurs. In each case, this is in comparison to free oscillations, i.e. in comparison to oscillations, with which the fork oscillates when it is free of contact with the medium. If, for example, the amplitude of the oscillations becomes smaller, this means that the medium is in contact with the tuning fork and that the medium has reached a fill level dependent on placement and construction of the tuning fork. If, in contrast, the amplitude becomes greater, then the fill level of the medium lies again beneath the level, whose reaching was previously indicated by the measuring device. This monitoring of the exceeding and subceeding (falling beneath) of a fill level permits application for so-called min or max protection, e.g. protection against running empty or protection against overflow. Particularly because of this monitoring function, such measuring devices are also referred to as switches.
The switches serve usually for safety, i.e. that overfilling does not occur, or that pumps do not run empty. Therefore, checks must be made, from time to time, on whether the measuring-devices/switches are functioning reliably. For this purpose, function tests are provided.